The present invention relates to a spindle motor capable of rotating at high speed, which employs hydrodynamic bearings as radial and thrust bearings. More particularly, the present invention relates to a spindle motor which is designed to rotate with minimal vibrations irrespective of the position of the motor when used and hence is suitable for a hard disk driver (hereinafter referred to as simply "HDD").
With the achievement of HDD's with a high storage capacity and low power consumption, demand has been made for improvements in the performance of spindle motors which are used to drive them so as to be even more suitable therefor.
FIG. 16 schematically shows a conventional spindle motor which is actually used in an HDD, and FIG. 15 is a partially sectioned elevational view of the conventional spindle motor. The spindle motor 20 has a shaft support cylinder 22 in the center of a mount 21. A stator coil 23 which comprises a plurality of electromagnetic coils is secured to the outer periphery of the shaft support cylinder 22. A rotary shaft 25 is rotatably supported by the inner periphery of the shaft support cylinder 22 by ball bearings 24. The rotary shaft 25 has a support member 27 secured to the upper end thereof, the support member 27 being arranged such that hard disks 30 are fixedly mounted on the outer peripheral surface thereof. The support member 27 has a plurality of rotor magnet members 28 secured to the inner peripheral surface in opposing relation to the stator coil 23.
In the above-described spindle motor employing ball bearings, the magnitude of vibrations of the spindle motor depends on the internal clearances of the ball bearings. The magnitude of vibrations in the radial direction is substantially equal to the radial internal clearance of the ball bearings. Similarly, the magnitude of vibrations in the thrust direction is substantially equal to the thrust internal clearance of the ball bearings. Measures have been taken to reduce these internal clearances, for example, by preloading the ball bearings. However, no satisfactory internal clearance value has heretofore been obtained, i.e., it has been only possible to achieve 0.5 microns or so in terms of the non-repeated component of the runout in the radial direction. In addition, preloading of ball bearings results in an increase in the required torque of the motor instead and hence retrogresses to the desirous lowering in the power consumption of the HDD. Accordingly, as long as ball bearings such as those described above are used, it is in principle virtually impossible to further reduce the vibrations of the spindle motor.
In addition, it is necessary to use small-sized ball bearings for the above-described spindle motor, which involves the problem that the HDD is not shock resistant and is therefore inferior in durability. With regard to the achievement of high speed, i.e., 3,600 rpm.fwdarw.5,400 rpm.fwdarw.6,400 rpm, the prior art suffers from the problem of how to minimize the wear of the bearings. Further, since the prior art uses a lubricating oil (grease), the degree of cleanliness is low.
In case of a spindle motor whose bearings are only replaced by hydrodynamic bearings, it still suffers from the disadvantage that rotational vibration increases when they are used in a horizontal position. In addition, since the clearance between a movable piece and a fixed piece of a radial bearing is of the order of microns, when two discrete radial bearings are used, it is difficult to align them concentrically. When two discrete thrust bearings are used, it is difficult to adjust the relative position between the thrust bearings. In addition, since the thrust collar, that is, movable piece, of a thrust bearing is produced so that the parallelism is within several microns, it is necessary to hold down the parallelism to about 1 micron when it is assembled, which is very difficult. In a radial gap type spindle motor wherein a rotor magnet member is disposed around the outer periphery of the stator coil with a radial gap provided therebetween, moment is generated due to the imbalance of radial magnetic force acting between the stator coil and the rotor magnet member, causing the axis of the rotor to be inclined with respect to the support shaft, which results in an increase in the starting torque because of local contact of the dynamic pressure surfaces. In rotation, unstable radial magnetic force causes whirling of the shaft and therefore makes it impossible to obtain a satisfactory operating condition.
It is an object of the present invention to provide a spindle motor which employs hydrodynamic bearings to improve the high-speed rotating performance and minimize the vibration irrespective of the position of the motor when used and which is therefore suitable for a high-recording capacity HDD.
The present invention provides a spindle motor which employs hydrodynamic bearings to improve the durability, clean operation and high-speed rotating performance and minimize vibrations when rotating irrespective of the position of the motor when used and which is therefore suitable for a high-recording capacity HDD.